The present invention is generally in the area of hydrocarbon synthesis, more particularly, the production of lube base stocks, preferably unconventional base oils, from Fischer-Tropsch or wax-derived olefins.
Oxygenates such as methyl t-butyl ether (MTBE) and tertiary-amyl methyl ether (TAME) have been part of the United States gasoline strategy since the late 1970s. These ethers are typically produced by reacting branched C4 and C5 olefins, respectively with alcohols in a catalytic distillation unit. Environmental concerns will likely result in the phase-out of these additives. However, oil companies have expended a great deal of capital developing MTBE and TAME plants to produce these oxygenates, and will need to find new methods for using the plants once MTBE and TAME production is phased out.
Lube base stocks are made by a variety of methods, including subjecting waxy feedstocks to wax isomerization and/or solvent dewaxing, and oligomerizing 1-decene. Yields from wax isomerization and/or solvent dewaxing are relatively low, and costs are relatively high. 1-Decene is a relatively expensive feedstock for lube base stock production.
It would be desirable to have additional methods for forming lube base stocks. It would also be desirable to have methods for using MTBE and TAME plants for other purposes, such as the production of lube base stocks. The present invention provides such methods.
Lube base stock compositions, and methods for preparing the compositions, are disclosed. The methods involve subjecting a predominantly C5-11 olefin-containing feedstock to dimerization conditions, preferably using nickel ZSM-5 as the dimerization catalyst, to give a first product where the majority of the olefins in the olefinic feed are converted to hydrocarbons predominantly in the C10-22 range. The first product, optionally combined with an olefin-containing feed in the C10-22 range, preferably in the C12-18 range, is subjected to an additional dimerization step, using the same or a similar dimerization catalyst, to provide a second product that includes hydrocarbons in the lube base stock range. The olefinic feedstock may include paraffins as well as olefins, which paraffins do not participate in the dimerization reactions. Accordingly, the second product includes relatively heavy hydrocarbons in the lube base stock range, as well as relatively light unreacted paraffins (and any unreacted olefins). The hydrocarbons in the lube base stock range can be readily separated, for example, via distillation.
In one embodiment, at least a portion of the olefin-containing feeds is derived, in whole or in part, from Fischer-Tropsch synthesis. Fischer-Tropsch products in the C5-11 range typically include about 30-60% by weight olefins when the synthesis is performed using a fixed or fluidized bed reactor.
The olefinic feeds may need to be pre-treated to avoid fouling various dimerization catalysts. If the feeds contain oxygenates such as alcohols and acids, the oxygenates can be removed, for example, by adsorption, extraction, dehydration and/or decarboxylation. Alternatively, they can be removed by hydrotreatment. However, hydrotreatment reduces olefins to paraffins, and the resulting paraffinic feed may need to be subjected to dehydrogenation conditions to form olefins. The dehydrogenation conditions tend to form diolefins, which can be removed via selective hydrogenation. The olefinic feed, the first product and/or the second product can be subjected to hydroisomerization conditions to control the pour and cloud points. The second product can be subjected to hydrofinishing conditions to hydrogenate any remaining olefins.
The second product is preferably separated from any unreacted paraffins in the olefinic feed, which do not participate in the dimerization reactions. These paraffins can be subjected to dehydrogenation conditions, and recycled through the dimerization steps.
Preferably, one or more of the dimerization steps are performed using a catalytic distillation unit or a fixed catalytic bed. More preferably, the method involves using the fixed bed in an MTBE or TAME plant to perform the first dimerization step, and/or the catalytic distillation unit from an MTBE or TAME plant to perform the second dimerization step and product isolation.
The resulting product includes hydrocarbons in the lube base stock range, which can optionally be further processed. The product can be combined with various additives to provide a finished lube composition. Preferably, the product includes sufficient branching to lower the pour and cloud points, but sufficient linear paraffinic character to maintain a relatively high viscosity index. More preferably, the product is an unconventional base oil. The viscosity indices of the unconventional base oils are much higher than those commonly used in the industry.